MRI is a widely practiced method for mapping diseases in the lower extremities. Optimizing the design of signal detection coil array for imaging the lower extremities from the pelvis to the feet is a challenging problem become of the long longitudinal extent and the wide variations of anatomical structures. Surface coil arrays described in U.S. Pat. No. 4,825,162 by Roemer et al have been applied to lower extremity coil array designs. These designs typically employ sections of coil elements in an anterior plate or a pair of anterior and posterior plates flexed to the body of the lower extremity and can be grouped into four types of geometries: single enclosure of both legs, anterior flex onto both legs, orthogonal separation between legs, and a pair of anterior-posterior plates. These geometries do not provide the closest possible distance between signal detection coil elements and the anatomic structures of interest.
The geometry of single enclosure of both legs is described in U.S. Pat. Nos. 5,548,218, 6,137,291, and 6,438,402. The flexion of a butterfly surface coil element into a loop enclosing the body is described in U.S. Pat. No. 5,548,218 to provide quadrature signal detection with in-depth and homogeneous sensitivity. U.S. Pat. No. 6,137,291 describes a telescopically tapered array of butterfly coils, which further improves detection SNR because of reduced distance between detection coil elements and anatomic parts. This design is improved by U.S. Pat. No. 6,438,402, which introduces multiple butterfly coils, further reducing the coil size and hence reducing detection noise. Yet because coil sensitivity is proportional to the inverse of the radius of the enclosure, the detection SNR of this singular enclosure of both legs is much less than that of an enclosure of a single leg. However, imaging with smaller diameter coils placed separately around each leg suffers from SNR reduction due to inductive coupling between the coils.
The geometry of an anterior flexible array on both legs is described in U.S. Pat. No. 6,300,761, which reduces the distance between detection coil elements and anatomic parts by flexion onto each leg separately. Coil coupling between the right and left legs can be a serious issue. To alleviate this problem, decoupling circuits such as described in U.S. Pat. Nos. 5,489,847 and 5,708,361 can be used to enable proper coil function in this geometry of separate flexions onto both legs, and posterior surface of the lower extremity was not used for signal detection.
The geometry of an orthogonal separation between right and left is presented in U.S. Pat. Nos. 5,430,378 and 5,500,596 to minimize coupling interference between coil elements on the right and left legs. The orthogonality is maintained by orthogonality of coil planes, which prevents use of close distance between coil elements and anatomic parts.
The simple geometry of a pair of anterior-posterior plates is described in U.S. Pat. No. 6,323,648 and in Leiner et al, “Use of a three-station phased array coil to improve peripheral contrast-enhanced magnetic resonance angiography,” J Magn Reson Imaging 2004; 20:417-425. This simple geometry is a straight-forward extension of the Roemer surface coil array design and has minimal optimization for improving SNR.
These surface coils all suffer from the problem of variable signal sensitivity which results in images where the artery may vary in signal intensity along its length even though there is no disease. This is a particularly difficult problem for post-processing the images to obtain maximum intensity projections or volume renderings which are generally very sensitive to image intensity variations. A superior coil from the signal homogeneity point of view is a volume coil or birdcage coil which tends to have favorable uniformity of signal intensity over the entire image. With two modes, quadrature detection is possible which reduces noise by the square root of two and thereby enhances SNR. However, these volume birdcage coils have several technical problems that limit their utility for peripheral MR angiography. Placing a separate birdcage coil around each leg causes a large loss in SNR from inductive coupling between the coils. On the other hand, a birdcage coil big enough to fit both legs tends to have poor SNR because of the large distance between tissue and coil for large sections of the coil. Another problem is that parallel imaging is not possible with birdcage coils. This results in either slower and lower resolution scans when using birdcage coils. These problems have been addressed by using conductive shielding in between the birdcage coils, but this solution reduces SNR, especially when the coils are close together such as the case for coils on right and left legs. Compared to surface coils, birdcage coils have lower SNR near the surface. However, this is not an issue for imaging arteries which tend to be deep to the skin surface.
Because of these challenges, there is a need for an improved peripheral vascular MR angiography coil which employs separate coils surrounding each leg to give high SNR, does not suffer signal loss from inductive coupling between the coils, provides a homogeneous signal or an inhomogenous signal with relatively greater sensitivity in the region of major arteries, and permits parallel imaging to reduce scan time and improve resolution.